The present invention relates to a micromechanical component, in particular an acceleration sensor or a rotational speed sensor including function components suspended movably above a substrate, and a corresponding manufacturing method for the micromechanical component.
Although it may be applied to any micromechanical components and structures, in particular to sensors and actuators, the present invention and the underlying problem are elucidated with reference to a micromechanical acceleration sensor that may be manufactured using silicon surface micromachining technology.
Acceleration sensors, in particular micromechanical acceleration sensors manufactured using surface or volume micromachining technology, have an increasing market share in the automotive equipment industry and are increasingly replacing the piezoelectric acceleration sensors customarily used.
Conventional micromechanical acceleration sensors normally operate so that a flexibly mounted seismic mass device, which is deflectable in at least one direction by an external acceleration, on deflection causes a change in the capacitance of a differential capacitor device that is connected to it. This change in capacitance is a measure of the acceleration.
German Published Patent Application No. 195 37 814 describes a method of manufacturing surface micromechanical sensors.
A first insulation layer of a thermal oxide (approximately 2.5 xcexcm thick) is first deposited on a silicon substrate. Then, a thin (approximately 0.5 xcexcm thick) polysilicon layer is deposited on this insulation layer. The polysilicon layer is subsequently doped from the gas phase (POCl3) and structured by a photolithographic process. This conducting polysilicon layer to be buried is thus subdivided into individual regions that are insulated from one another and function as conductors or as surface electrodes arranged vertically.
A second insulation layer is deposited above the layers applied previously. This insulation layer is made up of an oxide generated from the gas phase. In a photolithographic process, the top insulation layer is structured by introducing contact holes into the top insulation layer, through which the underlying polysilicon layer may be contacted.
Then, a thin polysilicon layer, which functions as a seed for subsequent deposition of silicon, is applied. This is followed in another step by deposition, planarization and doping of a thick polycrystalline silicon layer. The deposition occurs in an epitaxial reactor. Then, a structured metal layer is applied to the thick silicon layer.
The thick silicon layer is structured in another photolithographic process, in which a photoresist mask is applied to the top side of the layer to protect the metal layer in the subsequent etching. Then, plasma etching of the thick silicon layer is performed through openings in the photoresist mask according to the method described in German Published Patent Application No. 42 410 45, in which trenches having a high aspect ratio are produced in the thick silicon layer. These trenches extend from the top side of the thick silicon layer to the second insulation layer. The layer is thus subdivided into individual regions that are insulated from one another, unless they are interconnected by the buried conducting layer.
The two sacrificial layers are then removed through the trenches in the area of the freely movable structures of the sensor. The oxide layers are removed by a vapor etching method using media containing hydrofluoric acid according to a method described in German Published Patent Application No. 43 172 74 and German Published Patent Application No. 197 04 454.
However, there are disadvantages with the removal of the sacrificial layer by the hydrofluoric acid vapor etching method. With this etching method, it is very difficult to achieve a defined undercutting, i.e., the oxide is removed not only beneath the functional or freely movable sensor structures, but also above and beneath the buried polysilicon conductors. This requires very wide conductors because the possibility of lateral undercutting must be assumed. Due to the undercutting, no conductors are allowed to pass beneath the functional structure. Another disadvantage is corrosion of the metal layer due to the hydrofluoric acid vapors.
If the water content of the gas phase is too high, there may be sticking problems, i.e., the freely movable sensor elements may adhere to the substrate. Due to the limited oxide thickness (due to the deposition method) of the insulation layers, the distance between the functional structure and the substrate is also limited.
Since the hydrofluoric acid vapor etching method is not compatible with the materials used in CMOS technology, there may not be any integration of the sensor element and the analyzer circuit.
The micromechanical component according to an example embodiment of the present invention and a corresponding manufacturing method may have the advantage that both the buried conductors and the sacrificial layer beneath the freely movable structures may be made of the same layer. Therefore, fewer layers and photolithographic processes may be needed.
The present invention provides a layer structure and a corresponding method for manufacturing micromechanical components, e.g., acceleration sensors having a lateral sensitivity, sacrificial layer regions being made of the same material, e.g., polysilicon, as the buried conductor regions. A defined etching of the sacrificial polysilicon layer regions is achieved with the method according to the present invention, thus preventing undercutting of the buried conductor regions.
The method according to the present invention allows a simple method of manufacturing a sensor element using only method steps that are conventional in semiconductor technology. In addition, only a few layers and photolithography steps are necessary with the method according to the present invention.
The first micromechanical function layer and the second micromechanical function layer may include polysilicon layers.
According to another example embodiment of the present invention, the first through fourth insulation layers are oxide layers.
In removing the sacrificial layer, if the first micromechanical function layer is made of polysilicon and the insulation layers are oxide layers, then etching media based on fluorine compounds (e.g., XeF2, ClF3, BrF3, etc.) may be used. The etching media have a very high selectivity with respect to silicon dioxide, aluminum and photoresist. Due to this high selectivity, the polysilicon conductor regions that are not to be etched, in contrast with the sacrificial polysilicon layer regions, are sheathed with silicon dioxide. This prevents etching or undercutting of the polysilicon conductor regions.
This also permits a conductor to be guided beneath the freely movable structures. Since the buried polysilicon conductor regions are no longer being undercut, they may be narrower. Reproducible removal of the sacrificial polysilicon layer regions that is well-defined both laterally and vertically is possible with the sacrificial polysilicon layer technology described above. Due to the high selectivity of the etching medium with respect to silicon dioxide, it may be possible to implement a multilayer system of polysilicon conductors and insulation layers, and crossing of lines may also be possible. Large lateral undercutting widths may be feasible in sacrificial layer etching, so that the number of etching holes in the seismic mass may be reduced or omitted entirely. This yields an increase in the seismic mass.
Since the etching process for removing the sacrificial polysilicon layer occurs in the gas phase, no problems may exist with regard to corrosion and sticking. The silicon sacrificial layer technology may be compatible with materials used in CMOS technology, thus permitting integration of the sensor element and analyzer circuit.
It may be possible to structure the insulation layers by dry etching processes through the choice of layer thicknesses and sequence, thus eliminating wet etching processes and yielding improved process tolerances. The distance between the freely movable structure and the silicon substrate layer may be adjustable as needed through the thickness of the polysilicon layer.
According to another example embodiment of the present invention, the conductor regions and the sacrificial layer regions are provided through local implantation and subsequent photolithographic structuring.
According to yet another example embodiment of the present invention, contact holes for connecting the second micromechanical function layer to the conductor regions are provided in the second and third insulation layers.
According to even another example embodiment of the present invention, contact holes for connecting the second micromechanical function layer to the substrate are provided in the first, second and third insulation layers.